The present invention relates to a process for the treatment, using anaerobic fermentation, of effluents with a view to the removal of the organic matter and of the sulfates which they contain.
The degradation of the organic matter present in effluents using anaerobic fermentation is performed by a complex ecosystem whose final stage is a fermentation resulting in the production of methane; the efficiency of the purification to which these effluents are thus subjected depends, therefore, in particular on the effectiveness of this last stage.
The ecosystem consists of a mixture of commensal bacterial genera whose food chain consists of the organic matter, and this results in its progressive destruction. Three separate stages can be distinguished, these being generally simultaneous in the treatment plant, where the various microbial genera coexist simultaneously with matter in various stages of its degradation.
In a first stage, the complex molecules are hydrolyzed into simpler molecules called volatile fatty acids (or VFA) such as tactic acid, acetic acid, propionic acid or butyric acid.
The second stage, called acetogenesis, corresponds to the conversion of these higher organic acids into acetic acid. This stage, carried out by so-called acetogenic bacteria, produces hydrogen and carbon dioxide.
The third stage is the methanization in which the residual metabolites are converted into methane by methanogenic bacteria of two distinct species, one employing the hydrogen and the carbon dioxide, the other, so-called acetoclasts, employing acetic acid.
When, as is frequently the case, the effluents additionally contain sulfates, a so-called sulfate-reducing flora develops in their presence; it produces hydrogen sulfide (H.sub.2 S) and perturbs the operation of the purifying ecosystem in the following manner:
during the second stage the sulfate-reducing bacteria consume a proportion of the VFA, to the cost of the acetogenics, that is to say of the production of acetic acid and hence of methane. In addition, the H.sub.2 S produced inhibits the behavior of the acetogenics with the result that a proportion of the VFAs can escape degradation and remain in the liquid left at the end of the treatment;
sulfate-reducing bacteria are also harmful in the third stage, firstly because it consumes hydrogen and carbon dioxide to the disadvantage of the methanogens, and secondly in that the H.sub.2 S produced inhibits the behavior of the acetoclastic bacteria.
The presence of sulfates in the effluents intended to be purified by fermentation to methane consequently presents the risk of limiting the performance of the biological reactors in which this fermentation takes place.